Extended X-ray Absorption Fine Structure Analysis Research Articles

Microstructural properties and local atomic structures of cobalt oxide nanoparticles synthesised by mechanical ball-milling process

In this study, facile preparation of pure and nano-sized cobalt oxides particles was achieved using low-cost mechanical ball-milling synthesis route. Microstructural and morphological properties of synthesised products were characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD results indicated that the fabricated samples composed of cubic pure phase CoO and Co3O4 nanocrystalline particles with an average crystallite size of 37.2 and 31.8 nm, respectively. TEM images showed that the resulting samples consisted of agglomerates of particles with average diameter of about 37.6 nm for CoO and 31.9 nm for Co3O4. Phase purity of the prepared samples was further investigated due to their promising technological applications. Local atomic structure properties of the prepared nanoparticles were probed using synchrotron radiation-based X-ray absorption spectroscopy (XAS) including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). EXAFS data analysis further confirmed the formation of single-phase CoO and Co3O4 nanoparticles. In addition, structural properties of cobalt oxide nanoparticles were investigated by performing density functional theory calculations at B3LYP/TZVP level and Born–Oppenheimer molecular dynamics. Theoretical calculations for both prepared samples were found to be consistent with the experimental results derived from EXAFS analysis. Obtained results herein reveals that highly crystalline and pure phase CoO and Co3O4 nanoparticles can be synthesised using simple, inexpensive and eco-friendly ball-milling method for renewable energy applications involving fuel cells and water splitting devices.

Synthesis of highly coke resistant Ni nanoparticles supported MgO/ZnO catalyst for reforming of methane with carbon dioxide

Ni-nanoparticles supported on MgO promoted nanocrytalline zinc oxide catalyst was prepared by hydrothermal method in presence of cationic surfactant cetyltrimethylammonium bromide. The catalyst showed very good activity for the reforming of methane with carbon dioxide to produce synthesis gas, where H2/CO ratio was almost 1 and the catalyst showed no deactivation for more than 100h. The prepared catalyst was characterized using the analytical techniques like N2-physisorption studies, X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Temperature programmed desorption (TPD), Temperature programmed reduction (TPR), Temperature programmed oxidation (TPO), H2-chemisorpton, Thermo-gravimetric analysis (TGA), Inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), and Extended X-ray absorption fine structure (EXAFS). Transmission electron microscopy and H2-chemisorption analysis indicated that highly dispersed Ni nanoparticles with average size 5.7nm, present on ZnO when MgO was added with the catalyst. The strong Ni–ZnO interaction was evidenced from TPR and EXAFS analysis. The presence of highly dispersed Ni nanoparticles and strong metal support interaction enhanced the reduction behaviour of the Ni-MgO/ZnO catalyst. The presence of MgO increased the adsorption behaviour of CO2, enhanced the dissociation of CO2 and accelerated the carbon elimination.

中性子回折と放射光によるLa-Co置換フェライトの局所構造解析

The La-Co substituted magnetoplumbite type (M-type) hexagonal ferrites are known to have the higher magnetic crystalline anisotropy compared with conventional M-type Sr ferrites. The site occupation of cation in the crystal structure has been analyzed by neutron diffraction and EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy in order to clarify the correlation between the local structural change and the improvement of the magnetic anisotropy. The EXAFS analysis has suggested that Co ions were located in the 4f1, which showed good agreement with the results by neutron diffraction and XMCD analysis.

Fluorescence Extended X-Ray Absorption Fine Structure Study on Local Structures of Rare-Earth-Doped InGaGdN

Local structures around gadolinium atoms in rare-earth (RE)-doped InGaGdN thin films were studied by means of fluorescence extended X-ray absorption fine structure (EXAFS) measured at the Gd LIII-edges. The samples were doped with Gd in-situ during growth by plasma-assisted molecular beam epitaxy (PAMBE). Gd LIII-edge EXAFS signal from the GaGdN, GdN and Gd foil were also measured as reference. The X-ray absorption near edge structure (XANES) spectra around Gd LIII absorption edge of InGaGdN samples observed at room temperature indicated the enhancement of intensities with the increase of Gd composition. Further EXAFS analysis inferred that the Gd atoms in InGaN were surrounded by similar atomic shells as in the case of GaGdN with the evidence indicating majority of Gd atoms substituted into Ga sites of InGaGdN. A slight elongation of bond length for the 2nd nearest-neighbor (Gd–Ga) of sample with higher Gd concentration was also observed.

Synchrotron X-ray absorption spectroscopy study of the local atomic structures and cation ordering in perovskite- and spinel-type zinc stannate synthesized by co-precipitation method

Zinc stannate nanocrystalline powders with two different crystal structures were synthesized from two different initial Zn2+/Sn4+ ratios by a simple co-precipitation method. X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) techniques were used to investigate crystal structure, oxidation, and cation ordering of the after-calcined powders. XRD patterns clearly showed that the ZnSnO3 perovskite phase was obtained using an equal initial Zn2+/Sn4+ ratio of 1:1, while a double Zn2+ excess sample (2:1) produced the pure Zn2SnO4 spinel phase. The chemical shifts in Zn K- and Sn L3-edge XANES spectra supported that only Zn2+ and Sn4+ ions existed in these stannate samples. Moreover, the theoretical simulation of the Zn K-edge EXAFS experimental data also indicated a change in the cation ordering from orthorhombic, for perovskite ZnSnO3, to cubic for spinel Zn2SnO4. This is the first time that the local atomic structural characterization of zinc stannate powders derived from EXAFS analysis has been presented.

Differences in local structure around Co and Fe of theBiCo1−xFexO3system determined by x-ray absorption fine structure

The local structures around Co and Fe in ${\mathrm{BiCo}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ (BCFO) have been determined element-selectively by means of extended x-ray absorption fine structure (EXAFS) measurements at the Co and Fe $K$ edges. In the tetragonal (T) phase at $x\ensuremath{\le}0.63$, both Fe and Co ions are trivalent and locate in the ${\mathrm{O}}_{5}$ pyramidal coordination. The four in-plane Fe bonds with transverse oxygen (Fe-O2) split into $2\ifmmode\times\else\texttimes\fi{}2$ bonds in the monoclinic (M) phase at $0.63\ensuremath{\le}x\ensuremath{\le}0.73$ whereas the Co-O2 bonds do not show a split exceeding the resolution of our EXAFS analysis. This result is an element-specific interpretation of the polarization rotation in the M phase around the morphotropic phase boundary. The components of thermal vibration and static disorder are evaluated separately from the temperature dependence of the Debye Waller factor from $T=8\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ to 300 K. The static disorder of the Fe site is large compared to that of the Co site, thereby indicating that Fe is unstable in the ${\mathrm{O}}_{5}$ pyramidal coordination of the T and M phases. This study demonstrates that the instability of Fe ions plays an important role in the sequential structural distortion of the BCFO system.

Muffin-tin potentials in EXAFS analysis.

Muffin-tin potentials are the standard tool for calculating the potential surface of a cluster of atoms for use in the analysis of extended X-ray absorption fine-structure (EXAFS) data. The set of Cartesian coordinates used to define the positions of atoms in the cluster and to calculate the muffin-tin potentials is commonly also used to enumerate the scattering paths used in the EXAFS data analysis. In this paper, it is shown that these muffin-tin potentials are sufficiently robust to be used to examine quantitatively contributions to the EXAFS data from scattering geometries not represented in the original cluster.

Effect of fluctuation in Al incorporation on the microstructure, bond lengths, and surface properties of an Al x Ga1−x N epitaxial layer

Al x Ga1−x N (0.15 ≤ x ≤ 0.87) epilayers were grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The lattice deformation, chemical bonding states, and local electronic structure were characterized by a series of techniques, including X-ray diffraction (XRD), Raman scattering, X-ray photoelectron (XPS), and extended Xray absorption fine structure (EXAFS) spectroscopy. The XRD and Raman spectra revealed that substitution of Ga with Al in Al x Ga1−x N epilayers induces a significant contraction in the crystal lattice and a slight expansion of internal parameter u. The XPS measurements indicated the absence of remarkable disorder of the chemical bonds in AlGaN with a high Al content. The EXAFS analysis also demonstrated that the Ga-N and Ga-Al bond lengths are independent of the Al content, whereas the Ga-Ga bond length varies with Al content.

X-ray absorption spectroscopy study of Cu(II) coordination in the interlayer of montmorillonite

Cu(II) coordination in the interlayer of an expandable clay mineral montmorillonite is studied using X-ray absorption spectroscopy (XAS) along with electron paramagnetic resonance (EPR) and X-ray diffraction (XRD). Ab initio calculations are performed using FEFF code to reproduce the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) portion of the experimentally measured XAS spectra. Cu(II) coordination changes depending on the Cu(II) loading and hydration state of the interlayer. XRD shows that the Cu-saturated montmorillonite has d001-spacing values corresponding to the interlayer thickness of four and one water layer(s) for the hydrated and dehydrated interlayers, respectively. EPR shows that Cu(II) in the fully hydrated, unsaturated interlayer behaves similarly to free Cu(II) ion in a bulk aqueous solution, while Cu(II) forms a square planar complex in the dehydrated interlayer. Cu(II) in the fully hydrated, Cu-saturated montmorillonite has a characteristic singlet 1st derivative XANES spectrum. FEFF calculations show that this singlet feature originates from a quasi-regular octahedral coordination of water molecules around the interlayer Cu(II) atom. All other samples and model compounds including the dry Cu-saturated montmorillonite, wet and dry unsaturated montmorillonite, aqueous Cu(II), cupric nitrate salt (Cu(NO3)2·4.5H2O), and Cu(II) hydroxide precipitates have doublet 1st derivative XANES spectra. FEFF calculations suggest that the doublet features arise from an axially elongated octahedral coordination under the Jahn–Teller effect or square planar coordination. FEFF calculations of the EXAFS spectra as a function of the axial oxygen bond length demonstrate that a destructive interference between backscattering from equatorial oxygen (Oeq) and that from axial oxygen (Oax) atoms leads to an apparent coordination number (CN) less than six expected for the tetragonal coordination, with the farther, loosely bound axial oxygen atoms making a minor, yet negative contribution to the CN determined by the EXAFS analysis. This study shows that Cu(II) has interchangeable octahedral, tetragonal, and square planar coordinations in the interlayer of montmorillonite, depending on Cu(II) loading and degree of hydration. The quasi-regular octahedral coordination of the interlayer Cu(II) in montmorillonite is a new finding of this study.

Lead Stabilization in Soil Amended with Lime Waste: An Extended X-ray Absorption Fine Structure (EXAFS) Investigation

To determine Pb species in soils following the immobilization process, sequential extraction has been used despite the possibility of overestimating Pb species from unintended reactions during chemical extraction. Meanwhile, the application of extended X-ray absorption fine structure (EXAFS) has been shown to provide a more precise result than chemical extraction. In this study, the immobilization of Pb in contaminated soils treated with liming materials such as oyster shell (OS) or eggshell (ES) was evaluated with thermodynamic modelling and EXAFS analysis. Thermodynamic modelling by visual MINTEQ predicted the precipitation of Pb(OH)₂ in OS and ES treated soils. In particular, the values of saturation index (SI) for Pb(OH)₂ in OS (SI=0.286) and ES (SI=0.453) treated soils were greater than in the control soil (SI=0.281). Linear combination fitting (LCF) analysis confirmed the presence of C 12 H 10 O 14 Pb 3 (lead citrate, 44.7%) by citric acid from plant root, Pb-gibbsite (Pb adsorbed gibbsite, 26.4%), and Pb-kaolinite (Pb adsorbed kaolinite, 20.3%) in the control soil. On the other hand, Pb(OH)₂ (16.8%), Pb-gibbsite (39.3%), and Pb-kaolinite (25.6%) were observed in the OS treated soil and Pb(OH)₂ (55.2%) and Pb-gibbsite (33.8%) were also confirmed in the ES treated soil. Our results indicate that the treatment with OS and ES immobilizes Pb by adsorption of Pb onto the soil minerals as a result of the increase in soil negative charge and the formation of stable Pb(OH)₂ under high pH condition of soils.

Characterizing phosphorus speciation of Chesapeake Bay sediments using chemical extraction, 31P NMR, and X-ray absorption fine structure spectroscopy.

Nutrient contamination has been one of the lingering issues in the Chesapeake Bay because the bay restoration is complicated by temporally and seasonally variable nutrient sources and complex interaction between imported and regenerated nutrients. Differential reactivity of sedimentary phosphorus (P) pools in response to imposed biogeochemical conditions can record past sediment history and therefore a detailed sediment P speciation may provide information on P cycling particularly the stability of a P pool and the formation of one pool at the expense of another. This study examined sediment P speciation from three sites in the Chesapeake Bay: (i) a North site in the upstream bay, (ii) a middle site in the central bay dominated by seasonally hypoxic bottom water, and (iii) a South site at the bay-ocean boundary using a combination of sequential P extraction (SEDEX) and spectroscopic techniques, including (31)P NMR, P X-ray absorption near edge structure spectroscopy (XANES), and Fe extended X-ray absorption fine structure (EXAFS). Results from sequential P extraction reveal that sediment P is composed predominantly of ferric Fe-bound P and authigenic P, which was further confirmed by solid-state (31)P NMR, XANES, and EXAFS analyses. Additionally, solution (31)P NMR results show that the sediments from the middle site contain high amounts of organic P such as monoesters and diesters, compared to the other two sites, but that these compounds rapidly decrease with sediment depth indicating remineralized P could have precipitated as authigenic P. Fe EXAFS enabled to identify the changes in Fe mineral composition and P sinks in response to imposed redox condition in the middle site sediments. The presence of lepidocrocite, vermiculite, and Fe smectite in the middle site sediments indicates that some ferric Fe minerals can still be present along with pyrite and vivianite, and that ferric Fe-bound P pool can be a major P sink in anoxic sediments. These results provide improved insights into sediment P dynamics, particularly the rapid remineralization of organic P and the stability of Fe minerals and the ferric Fe-bound P pool in anoxic sediments in the Chesapeake Bay.

Configuration Determination of Transition Metal Complexes by Multiple Scattering EXAFS Analysis: A Case Study

Abstract A multiple scattering extended X-ray absorption fine structure (EXAFS) analysis of Ti(acac)2Cl2 is presented. With such an approach the configuration of the ligands in this complex can be identified due to the different multiple scattering events in the cis- and trans-configuration. A thorough evaluation of the effects of the different multiple scattering parameters (effective scattering path, scattering order, number of scattering atoms) in combination with the input structure is presented that reveals a complicated correlation between these parameters which could lead to wrong results with respect to the determined configuration. Finally, the optimal procedure for this particular purpose is defined and described.

Uranium and technetium interactions with wüstite [Fe1–xO] and portlandite [Ca(OH)2] surfaces under geological disposal facility conditions

AbstractIron oxides resulting from the corrosion of large quantities of steel that are planned to be installed throughout a deep geological disposal facility (GDF) are expected to be one of the key surfaces of interest for controlling radionuclide behaviour under disposal conditions. Over the lengthy timescales associated with a GDF, the system is expected to become anoxic so that reduced Fe(II) phases will dominate. Batch experiments have therefore been completed in order to investigate how a model reduced Fe-oxide surface (wüstite, Fe1–xO) alters as a function of exposure to aqueous solutions with compositions representative of conditions expected within a GDF. Additional experiments were performed to constrain the effect that highly alkaline solutions (up to pH 13) have on the adsorption behaviour of the uranyl (UO22+) ion onto the surfaces of both wüstite and portlandite [Ca(OH)2; representative of the expected cementitious phases]. Surface co-ordination chemistry and speciation were determined byex situX-ray absorption spectroscopy measurements (both X-ray absorption near-edge structure analysis (XANES) and extended X-ray absorption fine structure analysis (EXAFS)). Diffraction, elemental analysis and XANES showed that the bulk solid composition and Fe oxidation state remained relatively unaltered over the time frame of these experiments (120 h), although under alkaline conditions possible surface hydroxylation is observed, due presumably to the formation of surface hydroxyl complexes. The surface morphology, however, is altered significantly with a large degree of roughening and an observed decrease in the average particle size. Reduction of U(VI) to U(IV) occurs during adsorption in almost all cases and this is interpreted to indicate that wüstite may be an effective reductant of U during surface adsorption. This work also shows that increasing the carbonate concentration in reactant solutions dramatically decreases the adsorption coefficients for U on both wüstite and portlandite, consistent with U speciation and surface reactivity determined in other studies. Finally, the EXAFS results include new details about exactly how U bonds to this metal oxide surface.

Aggregation of nanoscale iron oxyhydroxides and corresponding effects on metal uptake, retention, and speciation: II. Temperature and time

The aggregation and growth of nanosized particles can greatly impact their capacity to sorb and retain dissolved metals, thus affecting metal fate and transport in contaminated systems. Aqueous suspensions of synthesized nanoscale iron oxyhydroxides were exposed to dissolved Zn(II) or Cu(II) and aged at room temperature (∼20°C), 50°C, and 75°C for timeframes ranging from 0 to 96h before sorbed metal ions were desorbed by lowering the suspension pH. Atomic absorption spectroscopic analysis of supernatants both before and after the desorption step determined how temperature and time affect macroscopic metal uptake and retention capacities. Extended X-ray absorption fine structure (EXAFS) spectroscopy analysis described the local binding environment of the sorbed/retained metals on the solid phase.With increasing aging temperature and time, the initial ∼5-nm oblong nanoparticles formed dense aggregates, lost reactive surface area, and retained progressively larger fractions of the initially-introduced Zn(II) and Cu(II) following the desorption step, with the copper species inhibiting the oriented aggregation of the nanoparticles into nanorods. Based on EXAFS analysis, the speciation of the sorbed metal species evolves with increasing time and temperature from surface-sorbed metal ions, which readily desorb and return to solution, to more strongly-bound, structurally-incorporated metal ions. These retained metals appear to associate intimately with the nanoparticle aggregates by substituting for iron in the nanoparticle lattice or by binding within nanoparticle aggregate pore spaces.

Aggregation of nanoscale iron oxyhydroxides and corresponding effects on metal uptake, retention, and speciation: I. Ionic-strength and pH

The capacity of nanosized particles to adsorb and sequester dissolved metals can be significantly impacted by the mechanism and extent of aggregation the particles have undergone, which in turn can affect the long-term fate and transport of potentially toxic metals in natural aqueous systems. Suspensions of monodisperse nanoscale iron oxyhydroxides were synthesized and subjected to increased pH (pH 8.0, 10.0) or ionic strength (0.1, 1.0M NaNO3) conditions to induce various states of aggregation prior to conducting macroscopic adsorption/desorption experiments with dissolved Cu(II) or Zn(II). The metal adsorption and retention capacities of the nanoparticle aggregates were compared to one another and to non-aggregated control nanoparticles, while the mode(s) of metal sorption to the nanoparticle surfaces were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy analysis.With increasing aggregation by both pH and ionic strength, the proportion of introduced zinc adsorbed to the iron oxyhydroxide nanoparticles progressively decreased from 45% on the monodispersed control particles to as low as 16% on the aggregates, while the proportion of introduced zinc retained upon desorption (obtained by lowering the suspension pH) increased from 7% on the control particles to as much as 17% on the aggregated particles. Copper exhibited a subtler trend of only slightly declining uptake (from 43% to 36%) and retention (from 35% to 30%) with increasing aggregation state. EXAFS analysis was consistent with the macroscopic results, showing relatively little change in Cu speciation between samples analyzed before and after the desorption step but significant increases in Zn–Fe interatomic distances and coordination numbers after desorption. This suggests the presence of both strongly- and weakly-bound zinc ions; the latter are likely affiliated with less stable, more distorted surface sorption sites and are thus more readily desorbed, resulting in the retention of zinc that is bound to more stable, less-distorted sorption sites. For both metals, inner-sphere bidentate sorption appears to dominate the sorption process to the nanoparticle aggregates, with potential structural incorporation into the aggregates themselves.

Relationship between the adsorption species of cesium and radiocesium interception potential in soils and minerals: an EXAFS study

This study examined the radiocesium (RCs) interception potential (RIP), cation exchange capacity (CEC), total organic carbon (TOC) content, and adsorption species in soils and minerals by using extended X-ray absorption fine structure (EXAFS) spectroscopy. The RIP related to Cs+ adsorption by frayed-edge site (FES) has often been used to measure the mobility and bioavailability of RCs in the environment. This study found that the presence of organic matter (OM) can reduce RIP to a certain extent. The adsorption amount (=QT) in soil was obviously correlated to RIP at a small [Cs+] region, whereas a linear relationship between QT and CEC was observed at a large [Cs+] region. Both the inner-sphere (IS) and outer-sphere (OS) complexes of Cs+ were observed through EXAFS at a molecular scale. The linear correlation between log (RIP/CEC) and the ratio of the coordination number (CN) of IS (=CNIS) and OS (=CNOS) complexes noted as CNIS/(CNIS + CNOS) suggested that the ratio of CN is very sensitive to Cs+ adsorption species with variable RIP and CEC. The adsorption species of Cs+ in soil was mainly dependent on the clay mineral content of soil. RIP was affected not only by FES but also by other strong adsorption sites, such as the interlayers and cavities identified as the IS complex in EXAFS analysis. Findings indicated that the EXAFS approach is a powerful and efficient tool to explore the behavior of Cs+ in a given environment.

Surface chemical and local electronic properties of Al(x)Ga(1-x)N epi-layers grown by MOCVD.

The surface chemical state and local electronic structure of Al(x)Ga(1-x)N (x = 0~0.45) epi-layers have been systematically investigated by X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The results show that the surface of Al(x)Ga(1-x)N is a composite of oxide and nitride of gallium and aluminum. In addition, it was identified that the Ga-O components were converted to Al-O components when the Al(x)Ga(1-x)N sample was exposed to air. The EXAFS analysis also reveals that the Ga-N and Ga-Al bond lengths are independent of the Al composition, whereas the Ga-Ga bond length is a function of Al composition.

EXAFS studies of novel impulsive hardening in the Be–Zn–Se semiconductor alloys

Beryllium doping has been used to harden the inherently soft zinc-selenides semiconductor mixed alloys. Stoichiometric semiconductor ternary alloys of BexZn1-xSe have been synthesized by the Bridgeman technique. Extended X-ray absorption fine structure (EXAFS) spectroscopy — in a state-of-art synchrotron-based method — is performed by varying the dopant Be concentration of Be from 6% to 55% in the zinc–selenide host semiconductor. EXAFS analyses is carried out to study the next neighbor and next nearest neighbor atomic positions, nature of the substitutional doping, extent of bond length homogeneity, presence of involuntary contrast among the path distances, and the cross over from soft to hard character of the ternary on increasing Be concentration. Our results indicate the presence of an impulsive nature of hardening in the ternary with a disparity at the lower and the higher doping levels. The observation of impulsive hardening of the substitutional dopants in the semiconductor lattice is explained by the help of self-accommodative attributes of the host lattice.

Enhanced Jahn-Teller response induced by low-dose 10 MeV I+ irradiation of La0.7Ca0.3MnO3−δ films

The structural response of pulsed laser deposited La0.7Ca0.3Mn03−δ films to low-level 10 MeV I+ irradiation is correlated with magnetic and electronic properties. A series of annealed pulsed laser deposited films were subjected to ion irradiation at doses of 0.5–2.0 × 1013 10 MeV iodine ions/cm2. X-ray diffraction measurements show no measurable increase in the full width at half maximum values of major reflections before and after irradiation. Extended x-ray absorption fine structure analyses (EXAFS), employing both a model independent fitting using expanded cumulants as well as a nonlinear least squares multiple scattering model based on FEFF generated theoretical EXAFS data, were employed to measure local properties relative to the Mn ions. Increasing irradiation doses lead to an unambiguous evolution of MnO6 octahedra from highly symmetric (i.e., undistorted) to a clear bimodal structure in which the MnO6 octahedra are measured to undergo a c-axis stretching bearing resemblance to a Jahn-Teller distortion. Concomitantly, the metal to insulator transition temperature decreases more than 270 K and the magnetoresistance increases from 102% to 106%.

An EXAFS study on the effects of natural organic matter and the expandability of clay minerals on cesium adsorption and mobility

The relationship between cesium (Cs) adsorption on clay minerals with various expandabilities and Cs mobility in environment was investigated using sequential extraction, batch adsorption, X-ray diffraction (XRD), generalized adsorption model (GAM), and Cs LIII-edge extended X-ray absorption fine structure (EXAFS) analyses with molecular simulations using the density functional theory (DFT). In particular, the difference between the affinities of illite (non-expansion) and vermiculite (intermediate expansion) for Cs and the effect of humic acid (HA) addition on the Cs/clay mineral system were highlighted in this study. These two factors affect Cs mobility and bioavailability in surface soil and sediments.The batch adsorption results showed that Cs adsorption was inhibited to some extent in the ternary clay+HA+Cs system because of (i) the blocked access of Cs to the frayed edge site (FES) and type II site [inner–sphere (IS) complex in GAM] by HA, and (ii) the reduced availability of the interlayer site in vermiculite. EXAFS analysis further confirmed that the adsorbed Cs in clay minerals was drastically changed by the sequential addition of HA. In addition, the dominant IS complex in the illite+Cs and illite+Cs+HA systems (in which HA was added after Cs adsorption on illite) can be converted to the outer–sphere (OS) complex largely in the illite+HA+Cs system (in which HA was added prior to Cs adsorption). These results are consistent with the sequential extraction and GAM results.The IS complex of dehydrated Cs+ mainly formed at the FES and interlayer site on illite (non-expansion) without resulting in any illite structural changes. However, on vermiculite (intermediate expansion), the dehydrated Cs+ can be adsorbed as an IS complex associated with the siloxane group of the di-trigonal cavity in the tetrahedral SiO4 sheet. This adsorption is accompanied by collapse of the layer, which can be easily coated by HA molecules to prevent Cs fixation. However, a nearly complete OS complex was observed at the planar site of montmorillonite (large expansion). These processes were confirmed by sequential extraction, batch adsorption, XRD, and EXAFS, which clearly showed that Cs mobility in soil highly depends on clay mineral expandability, natural organic matter (NOM), and the coupling of both effects. The atomic-scale information given by EXAFS is consistent with the distribution data from adsorption experiments, GAM, sequential extraction, and DFT. These results can be used as a basis for a clearer understanding of Cs behavior in natural systems.